Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with a device called an external programmer, or programmer, via a radio frequency telemetry link. A clinician may use such a programmer to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker may be modified after implantation in this manner. Modern implantable medical devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may be telemetered from an implantable medical device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations.
Telemetry systems for implantable medical devices may utilize radio frequency (RF) energy to enable bidirectional communication between the implantable medical device and an external programmer. In some applications, a wireless RF carrier is modulated with digital information, typically by amplitude shift keying where the presence or absence of pulses in the signal constitute binary symbols or bits. An exemplary telemetry system for an external programmer and a cardiac pacemaker is described in U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned to Cardiac Pacemakers, Inc., the disclosure of which is incorporated herein by reference. The external programmer transmits and receives the radio signal with an antenna incorporated into a wand that can be positioned in proximity to the implanted device. The implantable medical device also generates and receives radio signals by means of an antenna, which may include a wire coil inside of the device casing.
Some RF telemetry systems used for implantable medical devices, such as cardiac pacemakers, utilize inductive coupling between the antennas of the implantable medical device and an external programmer in order to transmit and receive wireless signals. Because the induction field produced by a transmitting antenna falls off rapidly with distance, such systems require close proximity between the implantable medical device and a wand antenna of the external programmer in order to work properly, usually on the order of a few inches. This requirement is inconvenient for the patient and the clinician, and thereby limits the situations in which telemetry can take place.